There is considerable interest in high power solid state lasers both for military and industrial applications. Some progress has been achieved through several approaches including slab lasers and heat capacity lasers for bulk solid-state lasers, and coherent fiber laser arrays for fiber based laser systems. The major problem confronting methods employing bulk solid-state materials is heat management. Thermal gradients cause the laser beam to deteriorate in quality and the system efficiency to deteriorate due to high temperature's negative impact on the inversion process.
The major problem confronting phased-array fiber lasers is the complexity associated with phasing a large number of single mode fibers employed in fiber phasing, especially when the number of fibers are in the hundreds. Employing single-mode fibers in these fiber arrays is necessary due to beam quality requirements. However, power scaling in single-mode fibers has been limited to few hundred watts per fiber.
On the other hand, using a multimode fiber amplifier for high power solid state lasers requires achieving diffraction-limited outputs. In previous attempts at achieving diffraction-limited output from multimode fiber amplifiers, higher order mode suppression has been the theme. Adjusting fiber index and dopant distribution, as described in H. L. Offerhaus, N. G. Broderick, D. J. Richardson, R. Sammut, J. Caplen, and L. Dong, Opt. Lett. 23, 1683 (1988) and J. M. Sousa, and O. G. Okhotnikov, Appl. Phys. Lett. 74, 1528 (1999), cavity configuration, as described in U. Griebner, R. Koch, H. Schonnagel, and R. Grunwald, Opt. Lett. 21, 266 (1996) and U. Griebner, and H. Schonnagel, Opt. Lett. 24, 750 (1999), launch conditions of the seed beam, as described in M. E. Fermann, Opt. Lett. 23, 52 (1998) and I. Zawischa, K. Plamann, C. Fallnich, H. Welling, H. Zellmer, and A. Tunnermann, Opt. Lett. 24, 469 (1999), and coiling the fiber, as described in J. P. Koplow, D. A. Kliner, and L. Goldberg, Opt. Lett. 25, 442 (2000), have been employed with limited degrees of success. For example, in coiling the fiber, the fundamental mode suffers losses as well as the higher order modes, thus lowering the efficiency of the system.